1. Field of the Invention
The present invention relates to a thermopile used as an infrared rays detector, and particularly to a thermopile improved in its measurement sensitivity by enlarging the temperature differences between its hot junction portions and cold junction portions thereof.
2. Description of the Prior Art
A thermocouple for infrared ray detection utilizes the so called Seebeck effect, i.e., an effect causing a thermoelectromotive force when two kinds of metals or semiconductors having different thermoelectric characteristics from each other are junctioned and given different temperatures at their different junction portions. FIG. 1a shows a thermopile constructed by connecting a plurality of thermocouples in series to obtain a large thermoelectromotive force.
This thermopile has a pattern support thin film 50 formed on a substrate, and has a plurality of first and second thermoelectric material layers 51, 53 respectively, formed by, for example, photoetching technology. These first and second thermoelectric material layers 51 and 53 are connected to each other in series so as to construct a plurality of thermocouples 55.
Each thermocouple 55 has a hot junction portion A and a cold junction portion B. When the hot junction portion A is warmed by incident infrared rays, thermoelectromotive force is generated by temperature differences between the hot junction portions A and cold junction portions B. Thus, the infrared rays can be detected b measuring the thermoelectromotive force.
Namely, the thermopile has a light receiving portion 57 having a large infrared absorption ratio on the pattern support thin film 50, and the hot junction portion A of each the thermocouple 55 is arranged in the proximity of the light receiving portion 57. As shown in FIG. 1b, the foresaid support thin film 50 is housed in a can 59. In a portion of the can 59 right above the light receiving portion 57, there is provided a filter 61 having substantially the same shape as the light receiving portion 57 so as to transmit only the infrared rays having longer wave length than a predetermined one. The infrared rays (incident light D' in the same drawing) transmitted through the filter 61 are irradiated on the light receiving portion 57 on the thin film 50 and absorbed therein. As a result, temperature the of the receiving portion 57 is elevated, and the generated heat warms the hot junction portions A arranged in the proximity of the receiving portion 57. Thereby, the temperature difference between the hot junction portion A and cold junction portion B is enlarged, and the thermoelectromotive force is generated. Then, the infrared rays are detected by measurement of the thermoelectromotive force.
When an arrangement is achieved such that the temperature difference between the hot junction portion A of each the thermocouple and the cold junction portion B becomes large, the sensitivity of a thermopile becomes high.
In conventional thermopiles, if only the infrared rays like the incident light D', as shown in FIG. 1b, are transmitted, only the hot junction portions A are warmed, and the temperature difference between the hot junction portion A and cold junction portion B becomes relatively large. However, actually, there are other infrared rays (incident light C') obliquely transmitted through the filter 61. Some of the incident light C' is irradiated not on the light receiving portion 57 but on the cold junction portions B directly, and thereby the temperature difference between the hot junction portion A and cold junction portion B becomes small. Thus, the measurement sensitivity is decreased. This problem is thought to be mainly caused by a plane structure of the conventional thermopiles.
In order to solve the problem, some devices condense light onto the light receiving portion as much as possible by using a mirror condenser, such as a concave mirror or optical cone, and placing the light receiving portion at the focus thereof (Japanese Patent Publication Disclosure No. 57-104827). However, these thermopiles also have plane structures so that a part of the incident light can reach the cold junction portions directly. Thus, there still remains the problem that the temperature difference between the hot junction portion and the cold junction portion becomes small. Accordingly, there has been desired a drastical improvement so as to enlarge the temperature difference between the hot-cold junction portions.
Moreover, since the conventional thermopile as shown in FIG. 1 has a plane structure as mentioned above, it is difficult to form patterns of the thermocouple 55 efficiently on the pattern support thin film 50. Namely, since so called dead spaces 63 having relatively large areas are inevitably produced on the film 50, and the receiving portion 57 is provided at the mid-portion on the thin film 50, it is not possible to form the first and second thermoelectric material layer 51, 53 on the entire surface of the thin film 50. Accordingly, the number of patterns of the thermocouple 55 formed on the pattern support thin film 50 is considerably restricted.